Filter devices, such as those disclosed in U.S. Pat. Nos. 4,964,984, 4,169,795, 4,052,315, 4,043,918, and in GB 2 075 364), are readily obtainable on the market in a multiplicity of embodiments. The structure of the filter elements, which filter impurities out of a fluid, is determined by the manufacturer. Filter elements of higher quality have a multilayer meshed structure which may rest at least on one supporting fabric to enable the structure to resist higher pressure or pressure peaks.
In principle, it is desirable to arrive at uniform flow behavior inside the filter space during the filtration process, while avoiding dead spaces. The requirements set for the desired behavior increase as the viscosity of the fluid increases, especially if liquefied polymer materials or polymer solutions are to be filtered.
U.S. Pat. No. 3,817,377 discloses a generic filter device, in particular one for separation of impurities from hot thermoplastic polymer material. In one embodiment of the conventional filter device shown in FIGS. 4 and 5 of that patent, a conical filter element subdivides a conical filter space into two conical subspaces. The conicities of the two subspaces extend constantly in opposite directions but to different extents, that is, with different cross-sections relative to one point of reference. The diameter of the filter element is measured from the filter outlet in the direction of the filter inlet more or less constantly. The resulting conicity of the filter element is referred to the longitudinal direction of the filter outlet. The bounding walls following the subspaces as housing elements have a conical orientation comparable to that of the filter element, so that the conventional solution applied for this purpose is complex in structure. Control of flow, which is unfavorable from the viewpoint of energy, is obtained because of the different cross-sectional patterns referred to one point of comparison, especially in the case of high-viscosity fluids.